Democratic National Committee staffer Seth Rich was mysteriously killed near his Washington, D.C. home in July, and despite a sweeping investigation police have yet to name any suspects or motives in his death.

Now, the late 27-year-old's grieving parents are calling on the public for help.

"You have two people who shot my son twice in the back. We're not going to stop until we find my son's murderer. We will find his murderer, but we need all of you," Rich's mother, Mary Rich, said during a press conference in D.C. Monday.

Mary Rich and her husband, Joel, who live in Omaha, Nebraska, announced that they are upping the reward for any information that could lead to an arrest from $25,000 to $125,000 — a sizeable $100,000 of which was put up by Republican lobbyist Jack Burkman.

DNC staffer Seth Rich, 27, was shot to death on July 10.

(Handout)

"D.C. is like a political fraternity, this could have happened to any one of us," Burkman said. "It could have happened to my Republican staff, it could have happened to my Democratic staff. I have people working for me and often in those same neighborhoods, so it's just something that tugs on your heart strings."

Rich was shot in the back while walking home from a bar in the Bloomingdale neighborhood in the early morning hours of July 10. Investigators suspect he was the victim of a botched robbery, but suspicion arose after it became clear that Rich still had his wallet, watch and cell phone on him when he was found.

WikiLeaks founder Julian Assange laid the brickwork for a slew of conspiracy theories after hinting over the summer that Rich could have been a source in his anti-secrecy organization's publication of 30,000 hacked DNC emails. He offered up $20,000 to anyone who could provide information leading to a conviction in the Rich case.

Mary Rich said, "We will find his murderer, but we need all of you."

Joel Rich also revealed that his son's girlfriend was on the phone with Seth just moments before he was killed.

"She heard some voices or something. She asked 'are you home?' He said, 'yeah, I'm a block away, I'm home' and he said 'I have to go' and put the phone back in his pocket and two minutes later he was shot," Joel Rich said somberly. "What happened in those two minutes we would love to know."